The invention relates to a loading structure comprising a fluid transfer boom for transfer of cryogenic liquids from a first storage structure to a vessel, the boom having a first arm and a second arm which are mutually connected at a first end via a swivel joint. The invention in particular relates to a loading structure for liquified natural gas.
A fluid transfer boom for use in such a loading structure is described in U.S. Pat. No. 4,022,498. In this patent a marine loading arm for transferring hydrocarbons from an on shore loading structure to a tanker is disclosed. On the loading structure a first arm of the boom is connected to a vertical supporting pipe via two swivel joints. The first arm is maintained in a generally vertical position by means of a counter weight and tensioning cables. At the end of the first arm a second arm is connected via a swivel joint such that the centre lines of both arms can define a plane in which the arms can be moved and the angle between the arms can be varied. The end part of the second arm which is to be coupled to a tanker comprises three swivel joints for rotation around three perpendicular axes.
The known transfer boom that is described in the above US-patent has as a disadvantage that relatively large and complex counter weights and tensioning cables are necessary to maintain the arms in their proper position. These may be subject to failure and intensive maintenance when used in the often harsh offshore environment. Furthermore, upon use of the known transfer boom for transfer of liquified natural gas (LNG), the LNG could escape from the transfer boom to the atmosphere, creating a potentially hazardous flammable and/or explosive environment.
It is therefore an object of the present invention to provide a loading structure which is particularly suitable for transfer of LNG, and which can be operated in a reliable and safe manner.
It is another object of the present invention to provide a loading structure having a fluid transfer boom suitable for offshore use, which is fully self-aligning when in use and which can be produced and maintained at low costs.
Hereto the loading structure according to the present invention is characterised in that a liquid natural gas duct is supported within the first and second arms, which form a gas tight housing around the liquified natural gas duct.
The transfer boom according to the present invention provides a redundant containment system wherein the LNG duct is supported by the structurally strong and self-supporting transfer boom which confines the natural gas in case of a leak in the inner LNG duct. The arms of the transfer boom shield the sensitive low temperature LNG fluid paths and swivel joints from the contact with the outer environment. Hereby the chances of mechanical and/or chemical damage to the LNG duct and its swivel joint, for instance by relative movements of the storage structure and a shuttle tanker or from the sea water, are reduced. The transfer boom according to the present invention can be used for loading LNG to and from an on shore storage structure or can be used offshore on a floating storage structure.
The outer walls of the arms may define a continuous fluid path between the second ends of the arms, such that gas may be drawn out and any LNG vapour may be recovered, re-liquified and transported through the LNG duct.
In one embodiment according to the present invention, the LNG duct is provided with an internal swivel joint at a position that corresponds with the swivel joint of the outer arms. The LNG duct is near its internal swivel joint connected to the internal wall of the outer arms. For instance at the position of the swivel joint, the LNG duct may be provided with deformable wall parts. Thereby the LNG ducts can follow the motions of the outer supporting arms while the deformable wall parts, which may comprise a bellow or a slip joint or a section of the duct made of flexible piping, allow for thermal expansion and contraction of the LNG ducts. The deformable wall parts function as alignment means to maintain the internal swivel joint of the LNG duct in a concentric position with respect to the swivel joint of the outer supporting arms.
The LNG duct may be placed in a concentric configuration with a vapour return duct. In one embodiment the vapour return duct comprises a non-concentric duct within each outer supporting arm, wherein the internal swivel comprises an outer toroidal LNG vapour chamber around the LNG duct. The toroidal LNG vapour chamber of the internal swivel has an inlet connected to an upstream vapour duct section and an outlet connected to a downstream vapour duct section. According to this construction, the vapour return ductxe2x80x94which has a higher temperature than the LNG ductxe2x80x94can be properly insulated from the colder LNG duct and from the hotter side walls of the outer supporting arms. Furthermore, upon leakage of the swivel joint of the LNG duct, the LNG will be confined in the surrounding toroidal swivel chamber of the vapour return duct.
The space within the outer supporting arms surrounding the LNG duct and the vapour return duct, may be filled with a non-flammable gas, such as an inert gas. In this way, the chances of the LNG vapour forming an explosive mixture with the outer atmosphere upon leakage from the LNG duct is reduced. For further containment of the LNG, a pressurised gas at a pressure above the pressure in the LNG duct or in the vapour return duct may be used, such as pressurised air or a pressurised inert gas.
For monitoring the integrity of the LNG duct and swivel, the supporting arms may be provided with a gas sampling opening in the wall thereof for sampling and analysing the gas for traces of hydrocarbons.
An embodiment of loading structure which is particularly suitable for LNG, but which may also be used for the transfer of other substances such as crude oil or oil products, is characterised in that the arms comprise at least seven swivel joints in total, each arm being rotatable around three perpendicular axes, the first arm being suspended from the storage structure in a generally vertical direction, wherein the second arm can extend between the end of the first arm and the vessel in a generally horizontal direction. The transfer boom according to the present invention provides a relatively simple self-supporting construction which can move in all directions due to the seven swivel joints. The transfer boom is suitable for offshore offloading operations between a floating storage structure and a tanker such as between a weathervaning storage vessel and a shuttle tanker, and can be used under sea conditions when wave and current induced motions of the storage structure and the vessel cause relative pitch, roll and yaw, heave surge and sway. Because the first arm is suspended from the storage structure and carries the second arm, the transfer boom is self supporting and can be easily manoeuvred during coupling, decoupling and retracting it to a parking position. By attaching a counterweight to the first end of the arms, the loading structure of the present invention forms an offshore mooring boom that exerts a restoring force on the shuttle tanker and which allowes for a quick disconnection in emergency situations, where in the horizontal arm will swing back to a substantially upright position which is out of the way of the shuttle tanker.
In a preferred embodiment, the swivel joints are of substantially similar construction. In this way construction and maintenance costs of the transfer boom can be reduced.
In a further embodiment of the loading structure according to the present invention, the first am comprises at its first and second ends substantially similar, generally unshaped piping structures comprising, relative the centre line of the arm, a 90xc2x0 bend and connected thereto a 180xc2x0 bend.
By using substantially similar u-shaped piping structures, the swivel joints of the first arm can be placed in vertical alignment below the suspension point of the arm, so that minimal bending moments are exerted on the swivel joints.
In a further embodiment each arm comprises a substantially similar mid-section comprising on one end a fixed flange and on the other end a substantially similar swivel joint. Upon breakdown of one of the arms, it can easily be replaced by a spare part that may be used for both first and second arms.